The present invention relates to a temperature control, it particularly relates to temperature control for fixing a picture on paper, to which the picture is to be transferred ( this is referred to as "transferee paper" hereinafter ), in a picture reproducing process. This picture reproducing process may be particularly used in a picture reproduction forming apparatus employing an electrophotographic method, for example, an optical printer such as a laser printer, a duplicator, or a facsimile machine.
Generally speaking, in a picture reproducing apparatus of an electrophotographic method, for example, a laser printer, a picture fixing unit is used. The picture fixing unit is used for fixing a picture on a transferee paper, which picture has been transferred from a surface of a photosensitive belt member by means of toner. A heat roller is used in the picture fixing unit for heating and pressing a made-from-toner-picture on transferee paper so as to fix a picture on the transferee paper.
In the picture fixing unit, the heat roller is heated by means of an internal-heater. A picture-fixing-temperature-controlling apparatus ( it is referred by "PFTC apparatus" hereinafter ) is used in the picture fixing unit for controlling a surface temperature of the heat roller at a predetermined temperature. The PFTC apparatus interrupts electric power supply to the heater by means of a thermostat or a temperature-controlled fuse ( referred by "TC fuse" hereinafter ) if the heat roller is heated excessively.
However, generally speaking, a thermostat and a TC fuse do not have a temperature-response-property such as to prevent the following phenomenon. The phenomenon is that a part of the picture fixing unit may be melted by heat, or smoke may be emitted from a part of the picture fixing unit by heat even if a thermostat or a TC fuse interrupts electric power supply to the heater. The phenomenon may take place because the thermostat or the TC fuse would not interrupt before the corresponding part of the picture fixing unit may be heated excessively.